Paper manufacture requires a source of cellulose fibers. Common raw materials as cellulose fiber sources are hardwoods and softwoods as well as those of annual vegetable origin, such as wheat and rice straw, hemp, jute, as well as annual or perennial reeds and grasses, and sugar canes which are often biennial and, in some climates, perennial. Rag materials, as well as recycled fibers, can also be used. However, wood has been, and continues to be, a primary source of cellulose fibers for paper production.
Before use, the wood or other cellulose containing raw material must be processed to release the cellulose fibers. This operation is referred to as “pulping”. At present, commercial pulping operations are of three principal types: mechanical, full chemical, and semichemical. Another method of pulping, known as sulfite pulping, involves chemical transformation of the lignin component of the fiber source resulting in chemically modified lignin to be solubilized in water.
Full chemical and semichemical pulping employ chemical reagents to effect separation of the cellulosic fibers from other components. Wood chips or other raw materials are cooked with suitable chemicals in aqueous solution, usually at elevated temperatures and pressures. The object is to dissolve the organic binders holding the cellulosic fibers, termed “lignins”, which vary widely but typically comprise around 20-30% of common raw woods, for example, along with other types of organic molecules, such as saccharide molecules, and other extraneous compounds, leaving the cellulose fibers intact. Although there is generally some resulting cellulose degradation, the objective can be realized to a commercially satisfactory degree through the use of a variety of chemical reagents. Pulp yields from wood using such processes usually exceed 40% of wood dry weight. The resulting liquid stream after pulping is generally referred to as wood pulping liquor, which includes lignin therein.
Lignins have been studied extensively and are believed to generally comprise the noncarbohydrate portion of the cell walls of plant materials. Originally, the lignin content of plant materials was defined as the residue after hydrolysis with strong acid following removal of waxes, tannins, and other extractives, including resins and tall oils. Lignins are amorphous, have high molecular weight, and are predominantly aromatic in structure. In general, the monomeric units comprising lignins can be referred to as p-hydroxycinnamyl alcohols. More specifically, according to The Merck Index, lignins comprise coniferyl, p-coumaryl, and sinapyl alcohols. Their precise composition vary with several factors including the method of isolation, and the species, age, and growing conditions, of the plant. Lignins are more or less completely removed from fiber by the digestion process of chemical pulping and enter the water phase of digestion pulping liquor in oxidized colloidal form.
Wood pulping liquors are typically concentrated by multieffect evaporation. Concentrated pulping (black) liquors from the dominant U.S. process, Kraft pulping, are burned to provide both process energy and nearly-complete recycle of inorganic pulping chemicals. Liquors from processes, such as acid sulfite pulping, where chemical recycle is limited are marketed as concentrates. If Kraft lignin is separated, it is typically precipitated from partially concentrated hot pulping liquors. Because lignin precipitates at a pH lower than that at which hemicellulose precipitates, hemicellulose is typically coprecipitated with lignin. This may be advantageous for some applications, such as asphalt additives. However, the presence of carbohydrate increases weight loss during firing of lignins at temperatures above 200° C. A high volatiles content is not desired for applications such as carbon fiber feedstocks, in which a predominantly lignin material is fired at high temperature.
Hemicellulose is the predominant dissolved carbohydrate in wood pulping streams. A hemicellulose can be any of several heteropolymers (matrix polysaccharides) present in almost all cell walls along with cellulose. Hemicelluloses include xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan. Currently, the forest products industry is funding research to evaluate methods for decreasing black liquor carbohydrates in several ways, most notably by stripping hemicellulose from wood chips by using a two-step pulping process. Hemicellulose and inorganics can also be precipitated from liquors by addition of alcohol(s) or ketones in water.
At present, the major use of hemicellulose in alkaline (Kraft and soda) pulping liquor is combustion to supply mill energy. A small amount of hardwood hemicellulose, which consists primarily of pentose (5-carbon) sugars, is currently recovered from spent sulfite pulping liquors as a feedstock for xylitol synthesis or biosynthesis. Xylitol is used as a sweetener and flavoring agent. In the sulfite pulping technologies (neutral sulfite semichemical, acid sulfite) common outside of the U.S., the need to develop uses and processing methods have resulted in the evaluation of modern separation processes for liquor components, such as lignosulfonates and hemicelluloses. These efforts often involve the use of ultrafiltration.
Several methods are known for removing carbohydrates from pulping liquors prior to lignin filtration. These methods include direct enzyme hydrolysis of hemicellulose, ion exchange stripping of hemicelluloses with acid moieties (oxidized sulfate compounds, carboxyl groups), acid hydrolysis of lignin, and selective precipitation of dissolved carbohydrates. The high hemicellulose content of commercial lignins and lignins precipitated from black liquors make direct enzyme hydrolysis of hemicellulose generally impractical, while the pH of black liquors (>pH 11) is higher than the effective pH ranges (typically pH 3 to 8.5 or, rarely, 9) of available commercial enzymes. At pH 5 to 8.5, where lignin exists as a precipitate or gel, many carbohydrate-hydrolyzing enzymes bind to lignin. The low levels of acid moieties in hemicellulose from lignin and black liquors preclude the effective use of ion exchange resin stripping. The potential for damage to the lignin structure (hydrolysis and depolymerization) by acid hydrolysis generally precludes the use of the acid hydrolysis technique.
Selective precipitation of carbohydrates dissolved in black liquors does significantly reduce volatiles in lignins produced from those black liquors. At room temperature and using either weak Kraft or alkaline pulped black liquor or commercial lignins dissolved to a similar concentration, carbohydrate precipitation could be accomplished by dropping the pH from 12 to 14 down to pH 9.5 to 10.5. However, the hemicellulose precipitates are very small, typically about 1 micron in size, and form voluminous soft gels which can be removed by centrifugation. Cleaner lignin could then be prepared by precipitation of the centrate at pHs ranging from 3.0 to 6.0. Precipitated lignin could then be redissolved and the processing steps repeated until the desired lignin (or hemicellulose) purity was obtained. This method produces high quality lignins and recovers high quality hemicellulose. However, the method is non-economical because three pH cycles were typically required.
The necessary features of black liquor processing additives for major industry generally comprise 1) little or no use of “non-process elements,” (such as magnesium, iron, aluminum, nickel, or cobalt) which could cause equipment corrosion or pulp discoloration, 2) limited use of additives (not to exceed 100 to 1000 ppm each of three or fewer additives per separation), and 3) where possible, additives should be materials tested and accepted for use in paper or in foods. What is needed is an economical method to improve separation of carbohydrates from pulping and hydrolysis liquors that satisfies all the above requirements.